Research and development activities on the long-distance optical transmission systems that use optical amplifiers as repeaters, have been actively performed in recent years. The enhancement of capacity by wavelength division multiplexing (WDM) intended for division-multiplexing a plurality of optical signals with different wavelengths into one optical fiber that forms a communications transmission line is considered to be a technique effective in providing Internet-based multimedia services, in particular. In these long-distance optical transmission systems, transmission speeds and transmission distances are greatly limited by the wavelength dispersion of the optical fiber. The wavelength dispersion is an event in which optical signals with different wavelengths are propagated through the optical fiber at different speeds. Since the optical spectra of the optical signals which have been modulated at high speed contain different wavelength components, these components are affected by the wavelength dispersion occurring during the propagation through the optical fiber and each component reaches a receiver at the different time of day. As a result, the optical signals suffer waveform distortion after being transmitted through the fiber.
The technique of dispersion compensation becomes important to suppress waveform deterioration due to such dispersion. Dispersion compensation is a method of canceling the wavelength dispersion characteristics of the optical fiber, and thus preventing waveform deterioration, by disposing at an optical transmitter, receiver, repeater, or the like, an optical element that has wavelength dispersion characteristics inverse to those of the optical fiber used as the transmission line. The research and development of the devices having inverse dispersion characteristics, such as dispersion-compensating fibers and optical fiber gratings (e.g., the grating described in Japanese Patent Laid-open No. 10-221658), have heretofore been conducted to obtain dispersion compensators. The above-mentioned optical element is one such example.
Dispersion tolerance indicates the range of the residual dispersion which satisfies certain transmission quality standards (i.e., the total amount of optical dispersion of the transmission line fiber and the dispersion compensator). Since dispersion tolerance decreases in inverse proportion to the second power of the bit rate of optical signals, the dispersion compensation technique becomes more important as the transmission speed increases. For example, in a 10-Gbps transmission system, the dispersion tolerance of optical signals is about 1000 ps/nm, so in consideration of the fact that the amount of dispersion of a single-mode fiber (SMF) is about 17 ps/nm/km, the system can transmit light only through a distance up to about 60 km without using the dispersion compensation technique. The dispersion tolerance in 40-Gbps transmission is as small as about 60 ps/nm, that is, about 1/16 of the above, and an SMF transmission distance achievable at this tolerance level is up to about 4 km.
The transmission distances of the current trunk transmission lines which use optical repeaters are from several tens of kilometers to several thousands of kilometers, and thus the amount of dispersion of the dispersion compensator used needs to be changed according to the particular transmission distance. The dispersion compensation method that has been adopted for a 10-Gbps transmission system, for example, is by, for instance, providing beforehand a dispersion compensator whose amount of dispersion can be fixed in increments from 100 ps/nm to several hundreds of ps/nm, and then determining the appropriate amount of dispersion according to transmission distance during installation. A typical method of constructing the dispersion compensator in this example is by using the dispersion compensation fiber having a wavelength dispersion of a sign inverse to that of the transmission line. Likewise, a dispersion compensator whose amount of dispersion compensation can be changed in increments from 10 ps/nm to several tens of ps/nm is considered to be necessary for a 40-Gbps transmission system. In addition, in this case, changes in the amount of dispersion compensation with changes in the temperature of the transmission line fiber cannot be ignored. Accordingly, a dispersion compensator with a variably controllable amount of dispersion becomes necessary.